This invention relates to systems for testing cable networks. More particularly, it relates to systems for testing the frequency response and dynamic range of signal paths.
Cable systems currently in use typically allow two way communications between the headend or distribution hubs and many remote points that may be houses. A headend is a collection point for both upstream and downstream signals. A distribution hub, which is sometimes used in large systems, is an intermediate point between the headend and the fiber nodes where the downstream signals from the headend are split and the upstream signals are combined. For the sake of this patent, the terms headend and distribution hub may be used interchangeably. One frequently employed architecture is hybrid fiber-coax (HFC). Forward direction, or downstream, signals are transmitted from the headend via optical fibers to fiber nodes. At the fiber node, the downstream transmission is converted from an optical signal to an electrical signal. The signal is distributed from the fiber node to a plurality of remote points, which may be homes, via coaxial cable by splitting. Amplification overcomes the losses of the cable and splitting devices. This portion of the network is referred to as a tree-and-branch system. The downstream frequency range is typically 54 to 550 MHz. Downstream signals have traditionally been analog television (TV) carriers. Digital carriers, such as digital audio, digital TV, cable telephone, and computer data, are increasingly being transported by the downstream system. The most common downsteam digital modulation technique is expected to be 64-QAM (quadrature amplitude modulation). The most common impairments expected on the downstream system are Gaussian, or random, noise and composite triple beat (CTB) distortion products.
In the return direction, or upstream, signals are transmitted from the remote points in the 5 to 42 MHz frequency band to the fiber node. The same passive devices that acted as splitters for downstream signals act as combiners for upstream signals. At the fiber optic node, the combined upstream electrical signals are converted to an optical signal for transmission to the headend. Forward and return signals typically travel inside the same coaxial cable in opposite directions. The use of diplex filters allows bidirectional travel inside a single coaxial cable. In the fiber optic bundle, forward and return signals commonly travel in opposite directions in different optic fibers. Upstream signals will typically be digital transmissions associated with services such as internet access, telephony, and polling set-top boxes for status information.
Downstream signals are typically transmitted continuously, while upstream signal transmissions, which may be originated by subscribers, are transmitted intermittently.
Cable technicians currently have sweep equipment that is used for measuring the magnitude portion of the frequency response for both upstream and downstream signal paths. The sweep equipment is typically designed to produce minimal objectionable interference with analog television signals. However, frequency response is a complex number, with both an magnitude and a phase component The problem with currently available sweep equipment is that it measures only the magnitude portion of the frequency response. Currently available sweep equipment does not measure a complex frequency response, which also includes the phase linearity of the signal path. Phase linearity is commonly expressed as group delay which is a derivative of phase with respect to frequency.
Group delay is created by filters such as diplex filters. Group delay can also be created by multipath distortion, or echoes. The multipath distortion may be created by reflections generated inside the cable plant by physical damage or water inside the cable. Multipath distortion is a serious concern in the return band because the reflections are not sufficiently attenuated by the loss of the cable, which is very low in the 5-42 MHz frequency band.
Simpler older sweep systems sweep at a very high speed so that the time that the swept signal spends in a single 6 MHz wide channel is short, thereby creating a small but noticeable interference. Some modern sweep systems use a stepped frequency approach to avoid visible interference with analog carriers. The modern sweep systems may be programmed to avoid or minimize interference with video or audio carriers. These systems do not provide any phase information about the signal; they only provide magnitude information.
There is a test system that can provide phase information about a number of cable channels. This system is called the Link Analyzer and is available from Hewlett Packard. The test system was originally designed to test microwave radio links. The system uses a swept signal that is frequency modulated as it is swept However, this system is very expensive and requires all carriers to be removed from the channel to perform the test.
One way known in the art to characterize the frequency response of a channel is to use a reference, or training, signal combined with digital signal processing techniques. The frequency response of the channel is characterized as an intermediate step in programming an adaptive equalizer. The function of the adaptive equalizer is to flatten the frequency response of the channel. The reference signal is sent by a transmitter in a quiet period while no other signals are using the frequency band, and received by a receiver. The acquired reference signal is processed with a stored reference signal that is free of impairments. This system is used by high-speed telephone modems to characterize telephone lines to increase data throughput. In the United States, this system is also used to remove the ghosts or multipath distortion from analog television signals by sending the training signal on line 19 of a vertical interval. A television receiver acquires the distorted reference signal and determines the frequency response of the channel as an intermediate step to programming an adaptive equalizer to cancel the echo, thereby improving video quality. If there are other signals using the channel when the reference signal is being sent, the energy from the other signals produces wrong equalizer settings. Thus the channel must be unoccupied while the adaptive equalizer is being programmed.
Training signals can be used at baseband or at RF (radio frequency). A baseband channel has a frequency range between DC (direct current) and some upper frequency. For example, video is a baseband signal with a frequency range such as DC-4.2 MHz. An RF channel is located between two frequencies, such as 54-60 MHz for television channel 2. If an RF channel is being tested, the training signal, which is normally a baseband signal, must be both up converted and down converted. Cable channels, such as downstream channels 2-W and upstream channels T-7 to T-12 are 6 MHz wide RF channels. The system of the present invention tests the upstream cable channels, which are group of 6 RF channels in the 5-42 MHz frequency range, as a single very wide baseband channel.
The unimpaired reference signal is typically pre-programmed at the time of manufacture into the receiver and stored in a memory, such as a read-only memory (ROM). It is not common practice to transport the receiver to the transmit site to load or capture an unimpaired burst test signal. In the cable testing application, providing a direct connection between the transmitter and receiver to capture an unimpaired burst test signal has two advantages. The first is that any imperfection in the frequency response of the transmitter or receiver will be canceled automatically. Another advantage is that any of a number of suitable burst test signals may be used.
The reference signals used ideally have the property of flat spectral energy so that any residual background noise will have a minimal effect on the frequency response data. Using flat spectral energy implies that communications in any frequency band that is in use at the time the reference signal is sent will be disturbed. It is common knowledge that the reference signal should not be sent while the channel is being used by data services because the data services will interfere with the reference signal and the reference signal will also interfere with the data services.
It has been discovered that the most common form of return band impairment is high speed bursts of noise that are typically short but powerful. The noise bursts typically last less than 10-20 microseconds but have been observed to last hundreds of microseconds. The noise bursts have most of their energy content concentrated between 5 and 15 MHz. The noise bursts are sometimes powerful enough to distort, or drive return active devices into a non-linear mode. The common sources of return noise bursts are the switching of electrical devices, such as inductive loads or motors with brushes. These noise bursts may enter the cable plant at shield breaks. Since these burst noise impairments are so prevalent, cable data modems are rapidly evolving to perform well in the presence of these impairments. There are at least three ways to adapt to burst noise in a signal path. One is to use forward error correction with interleaving to allow an error correcting code, such as a Reed Solomon code, to produce error-free data in the presence of the burst noise. Another method that is used for telephone transmissions is to use audio obfuscation. Errored voice packets are discarded and replaced with spectrally-similar information or noise to hide the audio impairment created by the missing packet. The third method is to automatically re-transmit the packets that have errors. Errored packets are detected with a cyclic redundancy check (CRC). The third method is used on the internet.
One problem facing technicians is discovering signal paths with insufficient dynamic range. Distortion is created when an active device, such as a laser transmitter or an amplifier, is over-driven into a non-linear region. When the active device is just above its maximum linear power level, the active device can be described as clipping and the signal path can be said to be above its clipping threshold. Thus it is desirable to know the clipping threshold, which is the power level at which distortion products exceed some tolerable limit. The onset of clipping is typically abrupt with laser diodes. On the downstream path, distortion products can be measured at any time since the transmissions are continuous. On the upstream path the transmissions are intermittent and unreliable, so this method is impractical. One method known in the art is to use random or Gaussian noise and a notch filter to detect distortion products. Detection is typically done by measuring a distortion power level with a spectrum analyzer. As the signal path begins to distort, distortion power begins to rise in the frequency band that was removed by the notch. Another method to detect the clipping threshold is to use sine waves which generate generating harmonics when above the clipping threshold. These methods, because of human measurement times, create interference with services.
It has been observed that some coaxial cable plant has poor field integrity due to corrosion, craft error or other problems. An earlier U.S. patent filing (Ser. No. 08/865,237), of which this patent filing is a continuation in part, describes the problem and a solution. Basically, a burst test signal is induced onto the sheath of the coaxial cable by magnetic coupling. The sheath current enters the inside of the coax at the shield break, and is captured on the center conductor of the coax. The hardware and software described in this patent application can perform a sheath integrity test when used in conjunction with a magnetic coupling device.
It is a purpose of this invention to measurea complex frequency response of a cable system by using a burst test signal that has similar spectral and temporal characteristics to commonly observed burst noise. It is also a purpose of this invention to use the same test equipment to determine the dynamic range of the cable plant. It is also a purpose of this invention to produce minimal disturbance with upstream services, allowing testing in a frequency band that is being used by digital services. It is also a purpose of this invention to use the same hardware and software to test the integrity of home wiring by performing a sheath integrity test.